Purification of vinylidene chloride



Patented Aug. 18, 1942 PURIFICATION OF VINYLmENE CHLORIDE Lee H.'Horsley, Midland.

Fred Lowell Taylor and Mich assignors to The Dow Chemical Com- Mich., acorporation of MichL 1 pm, Midland, l

No Drawing; Application May Serial No. 391,346

3 Claims. (11202-42) This invention concerns an improved method ofpurifying monomeric vinylidene chloride. 7

vinylidene chloride monomer, whether produced by passing chlorine andethylene chloride into a molten metal chloride bath according to themethod described in U. 5. Patent 2,121,548 or by reactingtrichloroethane with lime, is found to have certain impurities such as1.1.1- trlchloroethane, 1.1.2-trichloroethane, the cisandtrans-symmetrical dichloroethylenes, and even on occasion,trichloroethylene, vinyl chloride, ethylene chloride, and the like. Suchimpurities, even in minute quantities, adversely affeet thepolymerization of the product and give any polymer obtained undesirableproperties. They must therefore be removed if satisfactory polymericproducts are to be produced. Normal purification methods, as byfractional distillation, have been found to be unsatisfactory, since themonomer polymerizes in the fraetionating column.

It is, then, among the objects of the present invention to provide amethod of removing impurities from vinylidene chloride monomer even whenthese are present in minute quantities, whereby polymerization in thepurification apparatus is avoided and a purified product readilyadaptable to polymerization is obtained.

It has now been discovered that when mono- -meric vinylidene chloride ismixed with methanol, and the mixture is distilled, an azeotropic mixtureof the two materials is obtained and that the azeotrope leaves thehigher. boiling im purities as residue. Thus, by the continuous additionto the distillation zone of crude vinylidene chloride monomer andmethanol, continuous purification may be obtained and the residueconstituting the impurities may be drawn oil and discarded at intervals.The azeotropic proportions are approximately 6 per cent methanol byvolume and 94 per centmonomerie vinylidene chloride.

Briefly, the invention involves distilling an azeotrope consisting ofmonomeric vinylidene chloride and methanol, and separating thevinylidene chloride from the thus-obtained condensate.-

The following example will serve to further clarify the principle of theinvention and its operation.

A charge composed of 49.4 pounds of crude vinylidene chloride'monomer(containing such impurities as trichloroethane, ethylene chloride, smallproportions of symmetrical dichloroethylene and the like) and 6.75pounds of methanol was distilled 'the methanol into in a stainless steelfractionating column, coming over at a temperature of 30 -33.5 C. under900 The product The azeotropic distillate contained by volumeapproximately 6 per cent methanol and about 94 per cent of vinylidenechloride and was, further, found C. at 760 millimeters. This mixture waswashed with water to remove the methanol, and the pure monomer storedunder suitable conditions for future use.

The azeotropic distillation may be carried out under superatmospheric orsubatmospheric pressure as well as under normal pressures. higherpressures are generally found to be more desirable, since theneed for aspecial cooling medium in the condenser is then obviated,-

In operating the process according to the invention, an excess of themethanol is usually added to the crude monomer since such excess does noharm and is insurance that all the vinylidene chloride is included inthe azeotrope.

The part of the distilling column'which comes in contact with thevinylidene chloride monomer should be made of material which does notcontaminate the product. Among the desirable materials of constructionare nickel, glass, and stainless steel.

It is not intended to limit the invention to the purification ofvinylidene chloride monomer obtained from trichloroethane and crackingethylene chloride. Since the vinylidene chloride forms an azeotrope withmethanol, any compounds not combining with the azeotrope and boilingabove or below the point at which the azeotrope is distilled can beseparated from the monomer by the method of the invention. In

the case. of impurities, such as vinyl chloride,

which boil lower than monomeric vinylidene chloride, removal may be byevaporation or distillation either before or after introduction of theimpure monomer.

We claim:

1. In a method of purifying vinylidene chloride monomer, the steps ofdistilling monomericvinylidene chloride as an azeotrope from a mixturethereof with not less sate.

to have a boiling point of 27.5-28

The"

or by chlorinating than 6 percent of [its volume of methanol, andseparating the vinylidene chloride from the thus-obtained condenridemonomer, the steps which include, adding to the impure monomericvinylidene chloride sufllcient methanol to remove the monomer byazeotropic distillation, distilling the mixture to separate an azeotropecomposed of vinylidene chloride and methanol, and separating purifiedvinylidene chloride from the distillate.

' FRED LOWELL TAYLOR.

LEE H. HORSLEY.

